Lithium silicate glass ceramic and glass with monovalent metal oxide

ABSTRACT

Lithium silicate glass ceramics and glasses comprising specific oxides of monovalent elements are described which crystallize at low temperatures and are suitable in particular as dental materials.

This present application claims priority to and is a continuation application of U.S. application Ser. No. 14/001,180, filed Dec. 2, 2013, which is a National Stage of International patent application PCT/EP2012/070219, filed on Oct. 11, 2012, which claims priority to European patent application No. 11185334.7 filed on Oct. 14, 2011, the disclosures of which are incorporated herein by reference in their entirety.

The invention relates to lithium silicate glass ceramic and glass which comprise monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and are particularly suitable for use in dentistry, preferably for the preparation of dental restorations.

Lithium silicate glass ceramics are characterized as a rule by very good mechanical properties, which is why they have been used for a long time in the dental field and there primarily for the preparation of dental crowns and small bridges. The known lithium silicate glass ceramics usually contain as main components SiO₂, Li₂O, Al₂O₃, Na₂O or K₂O, and nucleating agents such as P₂O₅.

DE 24 51 121 describes lithium disilicate glass ceramics which contain K₂O and Al₂O₃. They are prepared from corresponding nuclei-containing starting glasses which are heated to temperatures of from 850 to 870° C. for the crystallization of lithium disilicate.

EP 827 941 describes sinterable lithium disilicate glass ceramics for dental purposes, which also contain K₂O or Na₂O in addition to La₂O₃. The lithium disilicate crystal phase is produced at a temperature of 850° C.

Lithium disilicate glass ceramics which contain K₂O and Al₂O₃ are known from EP 916 625. A heat treatment is carried out at 870° C. for the formation of lithium disilicate.

EP 1 505 041 describes lithium silicate glass ceramics containing K₂O and Al₂O₃, which, when lithium metasilicate is present as main crystal phase, can be very satisfactorily machined e.g. by means of CAD/CAM processes, in order to then be converted by further heat treatment at temperatures of from 830 to 850° C. into high-strength lithium disilicate glass ceramics.

EP 1 688 398 describes similar K₂O— and Al₂O₃-containing lithium silicate glass ceramics which additionally are substantially free from ZnO. A heat treatment at 830 to 880° C. is applied to them to produce lithium disilicate.

U.S. Pat. No. 5,507,981 describes processes for producing dental restorations and glass ceramics that can be used in these processes. These are in particular lithium disilicate glass ceramics which contain Al₂O₃ and usually either Na₂O or K₂O.

U.S. Pat. No. 6,455,451 relates to lithium disilicate glass ceramics which can also comprise Cs₂O in specific embodiments. However, in these embodiments the presence of significant quantities of Al₂O and BaO is also necessary. The production of the desired lithium disilicate crystal phase requires high temperatures of from 800 to 1000° C.

WO 2008/106958 discloses lithium disilicate glass ceramics for veneering zirconium oxide ceramics. The glass ceramics contain Na₂O and are produced by heat treatment of nuclei-containing glasses at 800 to 940° C.

WO 2009/126317 describes GeO₂-containing lithium metasilicate glass ceramics which also comprise K₂O and Al₂O₃. The glass ceramics are processed to form dental products primarily by machining.

WO 2011/076422 relates to lithium disilicate glass ceramics which also comprise K₂O and Al₂O₃ in addition to high levels of ZrO₂ or HfO₂. The crystallization of lithium disilicate takes place at temperatures of from 800 to 1040° C.

The known lithium disilicate glass ceramics have in common that they require heat treatments at more than 800° C. in order to effect the precipitation of lithium disilicate as main crystal phase. A large quantity of energy is therefore necessary to prepare them. Further, in the known glass ceramics the alkali metal oxides K₂O or Na₂O, as well as Al₂O₃ and BaO, are as a rule present as essential components which apparently are required for the production of the glass ceramics and in particular the formation of the desired lithium disilicate main crystal phase.

There is therefore a need for lithium silicate glass ceramics during the preparation of which the crystallization of lithium disilicate can be effected at lower temperatures. Further, it should also be possible to prepare them without the alkali metal oxides K₂O or Na₂O as well as Al₂O₃ and BaO, previously regarded as necessary, and they should be suitable in particular for the preparation of dental restorations primarily in view of their optical and mechanical properties.

This object is achieved by the lithium silicate glass ceramic according to any one of claim 1 to 15 or 18. Also a subject of the invention are the starting glass according to claim 16 or 18, the lithium silicate glass with nuclei according to claims 17 and 18, the process for the preparation of the glass ceramic and the lithium silicate glass with nuclei according to claims 19 and 20 as well as the use according to claims 21 and 22.

The lithium silicate glass ceramic according to the invention is characterized in that it comprises monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof.

It is preferred that the glass ceramic comprises the monovalent metal oxide or mixtures thereof in an amount of 0.1 to 17.0, in particular 1.0 to 15.0 and particularly preferably 1.5 to 8.0 wt.-%.

It is particularly surprising that the formation of the glass ceramic according to the invention with lithium disilicate as main crystal phase is also achieved in the absence of various components regarded as necessary for conventional glass ceramics, such as in particular K₂O, Na₂O as well as Al₂O₃ and BaO, even at very low and thus advantageous crystallization temperatures of about 700° C.

The glass ceramic according to the invention therefore preferably comprises less than 1.0, in particular less than 0.5 wt.-%, preferably less than 0.1 wt.-% K₂O. The glass ceramic is particularly preferably essentially free of K₂O.

Further, a glass ceramic is preferred which comprises K₂O, Na₂O and mixtures thereof in an amount of less than 1.0, in particular less than 0.5 and preferably less than 0.1 wt.-% and particularly preferably which is essentially free of K₂O and Na₂O.

Moreover, a glass ceramic which comprises less than 5.3, in particular less than 5.1, preferably less than 4.0 and particularly preferably less than 3.0 wt.-% Al₂O₃ is preferred. In a further preferred embodiment, the glass ceramic is essentially free of Al₂O₃.

In a further preferred embodiment, the glass ceramic comprises less than 3.8, in particular less than 3.6 and preferably less than 2.5 wt.-% BaO. The glass ceramic is particularly preferably essentially free of BaO.

A glass ceramic is also preferred which excludes lithium silicate glass ceramic comprising at least 6.1 wt.-% ZrO₂.

Further, a glass ceramic is also preferred which excludes lithium silicate glass ceramic comprising at least 8.5 wt.-% transition metal oxide selected from the group consisting of oxides of yttrium, oxides of transition metals with an atomic number of 41 to 79 and mixtures of these oxides.

The glass ceramic according to the invention preferably comprises 55.0 to 85.0, in particular 60.0 to 78.0 and preferably 62.0 to 77.0 wt.-% SiO₂.

It is also preferred that the glass ceramic comprises 9.0 to 20.0, in particular 9.0 to 17.0 and particularly preferably 12.0 to 16.0 wt.-% Li₂O.

It is further preferred that the molar ratio between SiO₂ and Li₂O is from 2.2 to 2.6, in particular from 2.3 to 2.5 and particularly preferably is about 2.4.

The glass ceramic according to the invention can also comprise a nucleating agent. It is preferred that a nucleating agent is present. P₂O₅ is particularly preferably used for this. The glass ceramic preferably comprises 0 to 12.0, in particular 1.0 to 12.0, preferably 2.0 to 9.0 and particularly preferably 2.5 to 7.5 wt.-% P₂O₅.

In a further preferred embodiment, the glass ceramic comprises at least one and preferably all of the following components:

Component wt.-% SiO₂ 55.0 to 85.0  Li₂O 9.0 to 17.0 Rb₂O and/or Cs₂O 0.1 to 15.0 P₂O₅ 0 to 12.0, preferably 1.0 to 12.0.

The glass ceramic according to the invention can moreover also comprise additional components which are selected in particular from oxides of divalent elements, oxides of trivalent elements, further oxides of tetravalent elements, further oxides of pentavalent elements, oxides of hexavalent elements, melt accelerators, colourants and fluorescent agents.

In particular, the alkaline earth metal oxides, preferably CaO, BaO, MgO, SrO or a mixture thereof and particularly preferably MgO are suitable oxides of divalent elements.

Suitable oxides of trivalent elements are in particular Y₂O₃. La₂O₃, Bi₂O₃ and mixtures thereof, and preferably Y₂O₃.

The term “further oxides of tetravalent elements” refers to oxides of tetravalent elements with the exception of SiO₂. Examples of suitable further oxides of tetravalent elements are TiO₂, SnO₂ and GeO₂.

The term “further oxides of pentavalent elements” refers to oxides of pentavalent elements with the exception of P₂O₅. Examples of suitable further oxides of pentavalent elements are Ta₂O₅ or Nb₂O₅.

Examples of suitable oxides of hexavalent elements are WO₃ and MoO₃.

A glass ceramic is preferred which comprises at least one oxide of divalent elements, at least one oxide of trivalent elements, at least one further oxide of tetravalent elements, at least one further oxide of pentavalent elements and/or at least one oxide of hexavalent elements.

Examples of melt accelerators are fluorides.

Examples of colourants and fluorescent agents are oxides of d- and f-elements, such as the oxides of Ti, V, Sc, Mn, Fe, Co, Ta, W, Ce, Pr, Nd, Tb, Er, Dy, Gd, Eu and Yb. Metal colloids, e.g. of Ag, Au and Pd, can also be used as colourants and in addition can also act as nucleating agents. These metal colloids can be formed e.g. by reduction of corresponding oxides, chlorides or nitrates during the melting and crystallization processes. The metal colloids are preferably present in the glass ceramic in an amount of 0.005 to 0.5 wt.-%.

In particular, the glass ceramic according to the invention comprises Ag₂O in an amount of 0.005 to 0.5 wt.-%.

The term “main crystal phase” used below refers to the crystal phase which has the highest proportion by volume compared with other crystal phases.

In one embodiment, the glass ceramic according to the invention comprises lithium metasilicate as main crystal phase. In particular the glass ceramic comprises more than 5 vol.-%, preferably more than 10 vol.-% and particularly preferably more than 15 vol.-% lithium metasilicate crystals, relative to the total glass ceramic.

In a further particularly preferred embodiment, the glass ceramic comprises lithium disilicate as main crystal phase. In particular the glass ceramic comprises more than 10 vol.-%, preferably more than 20 vol.-% and particularly preferably more than 30 vol.-% lithium disilicate crystals, relative to the total glass ceramic.

The lithium disilicate glass ceramic according to the invention is characterized by particularly good mechanical properties and can be produced e.g. by heat treatment of the lithium metasilicate glass ceramic according to the invention. However, it can be formed in particular by heat treatment of a corresponding starting glass or of a corresponding lithium silicate glass with nuclei.

It has surprisingly been shown that the lithium disilicate glass ceramic according to the invention has very good mechanical and optical properties even in the absence of components regarded as essential for conventional glass ceramics. The combination of its properties even allows it to be used as dental material and in particular as material for the preparation of dental restorations.

The lithium disilicate glass ceramic according to the invention has in particular a fracture toughness, measured as K_(IC) value, of at least about 2.0 MPa·m^(0.5) and in particular at least about 2.3 MPa·m^(0.5). This value was determined using the Vickers method and calculated using Niihara's equation. Further, it has a high biaxial flexural strength of preferably from 400 to 700 MPa. Moreover, it displays a high chemical resistance ascertained by mass loss after storage in acetic acid. The chemical resistance is in particular less than 100 μg/cm². The biaxial flexural strength and the chemical resistance were determined according to ISO 6872 (2008).

The invention also relates to a lithium silicate glass with nuclei that are suitable for forming lithium metasilicate and/or lithium disilicate crystals, wherein the glass comprises the components of the above-described glass ceramics according to the invention. Thus this glass comprises monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof. For preferred embodiments of this glass reference is made to the preferred embodiments described above of the glass ceramics according to the invention.

The glass with nuclei according to the invention can be produced by heat treatment of a correspondingly composed starting glass according to the invention. The lithium metasilicate glass ceramic according to the invention can then be formed by a further heat treatment, and in turn be converted into the lithium disilicate glass ceramic according to the invention by further heat treatment, or the lithium disilicate glass ceramic according to the invention can also preferably be formed directly from the glass with nuclei. The starting glass, the glass with nuclei and the lithium metasilicate glass ceramic can consequently be regarded as precursors for the production of the high-strength lithium disilicate glass ceramic.

The glass ceramics according to the invention and the glasses according to the invention are present in particular in the form of powders, granulates or blanks, e.g. monolithic blanks, such as platelets, cuboids or cylinders, or powder compacts, in unsintered, partly sintered or dense-sintered form. They can easily be further processed in these forms. They can, however, also be present in the form of dental restorations, such as inlays, onlays, crowns, veneers, facets or abutments.

The invention also relates to a process for the preparation of the glass ceramic according to the invention and the glass with nuclei according to the invention, in which a correspondingly composed starting glass, the glass with nuclei according to the invention or the lithium metasilicate glass ceramic according to the invention is subjected to at least one heat treatment in the range of 450 to 950° C., in particular 450 to 800 and preferably 450 to 750° C.

The starting glass according to the invention therefore comprises monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof. In addition, it preferably also comprises suitable amounts of SiO₂ and Li₂O in order to allow the formation of a lithium silicate glass ceramic, and in particular a lithium disilicate glass ceramic. Moreover, the starting glass can also comprise still further components, such as are given above for the lithium silicate glass ceramic according to the invention. All those embodiments are preferred for the starting glass which are also given as preferred for the glass ceramic according to the invention.

In the process according to the invention, the glass with nuclei is usually prepared by means of a heat treatment of the starting glass at a temperature of in particular 480 to 560° C. The lithium disilicate glass ceramic according to the invention is then preferably produced from the glass with nuclei through further heat treatment at usually 600 to 750 and in particular 650 to 750° C.

Thus, much lower temperatures are used according to the invention for the crystallization of lithium disilicate than with the conventional lithium disilicate glass ceramics. The energy thus saved represents a clear advantage. Surprisingly, this low crystallization temperature is also possible in the absence of components, such as K₂O and Al₂O₂ as well as BaO, regarded as essential for conventional glass ceramics.

To prepare the starting glass, the procedure is in particular that a mixture of suitable starting materials, such as carbonates, oxides, phosphates and fluorides, is melted at temperatures of in particular from 1300 to 1600° C. for 2 to 10 h. To achieve a particularly high homogeneity, the obtained glass melt is poured into water in order to form a glass granulate, and the obtained glass granulate is then melted again.

The melt can then be poured into moulds to produce blanks of the starting glass, so-called solid glass blanks or monolithic blanks.

It is also possible to put the melt into water again in order to prepare a granulate. This granulate can then be pressed, after grinding and optionally addition of further components, such as colourants and fluorescent agents, to form a blank, a so-called powder compact.

Finally, the starting glass can also be processed to form a powder after granulation.

The starting glass, e.g. in the form of a solid glass blank, a powder compact or in the form of a powder, is then subjected to at least one heat treatment in the range of 450 to 950° C. It is preferred that a first heat treatment is initially carried out at a temperature in the range of 480 to 560° C. to prepare a glass according to the invention with nuclei which are suitable for forming lithium metasilicate and/or lithium disilicate crystals. This first heat treatment is preferably carried out for a period of 10 min to 120 min and in particular 10 min to 30 min. The glass with nuclei can then preferably be subjected to at least one further heat treatment at a higher temperature and in particular more than 570° C. to effect crystallization of lithium metasilicate or lithium disilicate. This further heat treatment is preferably carried out for a period of from 10 min to 120 min, in particular 10 min to 60 min and particularly preferably 10 min to 30 min. To crystallize lithium disilicate, the further heat treatment is usually carried out at 600 to 750, preferably 650 to 750 and quite particularly preferably 700 to 750° C.

In a preferred embodiment of the process, therefore

-   -   (a) the starting glass is subjected to a heat treatment at a         temperature of 480 to 560° C. in order to form the glass with         nuclei, and     -   (b) the glass with nuclei is subjected to a heat treatment at a         temperature of 700 to 750° C. in order to form the glass ceramic         with lithium disilicate as main crystal phase.

The duration of the heat treatments carried out in (a) and (b) is preferably as given above.

The at least one heat treatment carried out in the process according to the invention can also take place during hot pressing or sintering-on of the glass according to the invention or the glass ceramic according to the invention.

Dental restorations, such as bridges, inlays, onlays, crowns, veneers, facets or abutments, can be prepared from the glass ceramics according to the invention and the glasses according to the invention. The invention therefore also relates to their use for the preparation of dental restorations. It is preferred that the glass ceramic or the glass is shaped into the desired dental restoration by pressing or machining.

The pressing is usually carried out under increased pressure and increased temperature. It is preferred that the pressing is carried out at a temperature of 700 to 1200° C. It is further preferred to carry out the pressing at a pressure of 2 to 10 bar. During pressing, the desired shape change is achieved by viscous flow of the material used. The starting glass according to the invention and in particular the glass with nuclei according to the invention, the lithium metasilicate glass ceramic according to the invention and the lithium disilicate glass ceramic according to the invention can be used for the pressing. The glasses and glass ceramics according to the invention can be used in particular in the form of blanks, e.g. solid blanks or powder compacts, e.g. in unsintered, partly sintered or dense-sintered form.

The machining is usually carried out by material removal processes and in particular by milling and/or grinding. It is particularly preferred that the machining is carried out within the framework of a CAD/CAM process. The starting glass according to the invention, the glass with nuclei according to the invention, the lithium metasilicate glass ceramic according to the invention and the lithium disilicate glass ceramic according to the invention can be used for the machining. The glasses and glass ceramics according to the invention can be used in particular in the form of blanks, e.g. solid blanks or powder compacts, e.g. in unsintered, partly sintered or dense-sintered form. The lithium metasilicate glass ceramic according to the invention and lithium disilicate glass ceramic according to the invention are preferably used for the machining. The lithium disilicate glass ceramic can also be used in a not fully crystallized form which was produced by heat treatment at a lower temperature. This has the advantage that an easier machining, and thus the use of simpler equipment for the machining, is possible. After the machining of such a partly crystallized material, the latter is usually subjected to a heat treatment at a higher temperature and in particular 650 to 750° C. and preferably about 700° C. in order to effect further crystallization of lithium disilicate.

In general, after the preparation of the dental restoration shaped as desired by pressing or machining, it can also in particular be heat-treated in order to convert the precursors used, such as starting glass, glass with nuclei or lithium metasilicate glass ceramic, into lithium disilicate glass ceramic or to increase the crystallization of lithium disilicate or to reduce the porosity, e.g. of a porous powder compact used.

However, the glass ceramic according to the invention and the glass according to the invention are also suitable as coating material of e.g. ceramics and glass ceramics. The invention is therefore also directed towards the use of the glass according to the invention or the glass ceramic according to the invention for coating in particular ceramics and glass ceramics.

The invention also relates to a process for coating ceramics and glass ceramics, in which the glass ceramic according to the invention or the glass according to the invention is applied to the ceramic or glass ceramic and is exposed to increased temperature.

This can take place in particular by sintering-on and preferably by pressing-on. With sintering-on, the glass ceramic or the glass is applied to the material to be coated, such as ceramic or glass ceramic, in the usual way, e.g. as powder, and then sintered at increased temperature. With the preferred pressing-on, the glass ceramic according to the invention or the glass according to the invention is pressed on, e.g. in the form of powder compacts or monolithic blanks, at an increased temperature of e.g. 700 to 1200° C., applying pressure, e.g. 2 to bar. The methods described in EP 231 773 and the press furnace disclosed therein can be used in particular for this. A suitable furnace is e.g. the Programat EP 5000 from Ivoclar Vivadent AG, Liechtenstein.

It is preferred that, after conclusion of the coating process, the glass ceramic according to the invention is present with lithium disilicate as main crystal phase, as such glass ceramic has particularly good properties.

Because of the above-described properties of the glass ceramic according to the invention and the glass according to the invention as its precursor, they are suitable in particular for use in dentistry. A subject of the invention is therefore also the use of the glass ceramic according to the invention or the glass according to the invention as a dental material and in particular for the preparation of dental restorations or as a coating material for dental restorations, such as crowns, bridges and abutments.

Finally, the glasses and glass ceramics according to the invention can also be mixed together with other glasses and glass ceramics in order to produce dental materials with properties set as desired. Compositions and in particular dental materials which comprise the glass according to the invention or the glass ceramic according to the invention in combination with at least one other glass and/or one other glass ceramic therefore represent a further subject of the invention. The glass according to the invention or the glass ceramic according to the invention can therefore be used in particular as main component of an inorganic-inorganic composite or in combination with a plurality of other glasses and/or glass ceramics, wherein the composites or combinations can be used in particular as dental materials. The combinations or composites can particularly preferably be present in the form of sintered blanks. Examples of other glasses and glass ceramics for the preparation of inorganic-inorganic composites and of combinations are disclosed in DE 43 14 817, DE 44 23 793, DE 44 23 794, DE 44 28 839, DE 196 47 739, DE 197 25 553, DE 197 25 555, DE 100 31 431 and DE 10 2007 011 337. These glasses and glass ceramics belong to the silicate, borate, phosphate or aluminosilicate group. Preferred glasses and glass ceramics are of SiO₂—Al₂O₃—K₂O type (with cubic or tetragonal leucite crystals), SiO₂—B₂O₃—Na₂O type, alkali-silicate type, alkali-zinc-silicate type, silicophosphate type, SiO₂—ZrO₂ type and/or lithium-aluminosilicate type (with spodumene crystals). By mixing such glasses or glass ceramics with the glasses and/or glass ceramics according to the invention, for example the coefficient of thermal expansion can be set as desired in a broad range of from 6 to 20·10⁻⁶ K⁻¹.

The invention is explained in more detail below by means of examples.

EXAMPLES Examples 1 to 16—Composition and Crystal Phases

A total of 16 glasses and glass ceramics according to the invention with the composition given in Table I were prepared by melting corresponding starting glasses followed by heat treatment for controlled nucleation and crystallization.

For this, the starting glasses weighing from 100 to 200 g were first melted from customary raw materials at 1400 to 1500° C., wherein the melting was very easily possible without formation of bubbles or streaks. By pouring the starting glasses into water, glass frits were prepared which were then melted a second time at 1450 to 1550° C. for 1 to 3 h for homogenization.

In the case of Examples 1 to 9 and 11 to 16, the obtained glass melts were then poured into preheated moulds in order to produce glass monoliths. All glass monoliths proved transparent.

In the case of Example 10, the obtained glass melt was cooled to 1400° C. and converted to a finely divided granulate by pouring into water. The granulate was dried and ground to a powder with a particle size of <90 μm. This powder was moistened with some water and moulded to form a powder compact at a pressure of 20 MPa.

The glass monoliths (Examples 1-9 and 11-16) as well as the powder compact (Example 10) were then converted by thermal treatment to glasses and glass ceramics according to the invention. The thermal treatments used for controlled nucleation and controlled crystallization are also given in Table I. The following meanings apply

-   -   T_(N) and t_(N) Temperature and time used for nucleation     -   T_(C) and t_(C) Temperature and time used for crystallization of         lithium disilicate or lithium metasilicate

It can be seen that a first heat treatment in the range of from 480 to 510° C. resulted in the formation of lithium silicate glasses with nuclei and these glasses crystallized in the case of Examples 1-10 and 12 by a further heat treatment already at 700 to 750° C. and in particular 700° C. to glass ceramics with lithium disilicate as main crystal phase, as was established by X-ray diffraction tests. The heat treatment at a temperature of only 660 to 680° C. resulted in the case of Examples 11 and 13-16 in the formation of glass ceramics with lithium metasilicate as main crystal phase.

The produced lithium disilicate glass ceramics had high fracture toughness values, measured as critical stress intensity factor K_(IC), of more than 2.0 MPa·m^(0.5).

The biaxial strength GB was also high, at at least 480 MPa. It was determined according to dental standard ISO 6872 (2008) on test pieces that were prepared by machining of the respective lithium disilicate glass ceramic. A CEREC-InLab machine (Sirona, Bensheim) was used for machining.

The produced lithium disilicate glass ceramics and lithium metasilicate glass ceramics were able to be very satisfactorily machined in a CAD/CAM process or hot pressed into the form of various dental restorations, which were also provided with a veneer if required.

They were also able to be applied by hot pressing as coatings onto in particular dental restorations, e.g. in order to veneer the latter as desired.

Example 17—Hot Pressing of Glass with Nuclei

A glass with the composition according to Examples 6 and 7 was prepared by mixing corresponding raw materials in the form of oxides and carbonates for 30 min in a Turbula mixer and then melting the mixture at 1450° C. for 120 min in a platinum crucible. The melt was poured into water in order to obtain a finely divided glass granulate. This glass granulate was melted again at 1530° C. for 150 min in order to obtain a glass melt with particularly high homogeneity. The temperature was reduced to 1500° C. for 30 min and cylindrical glass blanks with a diameter of 12.5 mm were then prepared by pouring into preheated, separable steel moulds or graphite moulds. The obtained glass cylinders were then nucleated in the range of from 480-560° C., depending on the composition, and stress-relieved.

The nucleated glass cylinders were then processed by hot pressing at a pressing temperature of 900-1100° C. using an EP600 press furnace, Ivoclar Vivadent AG, to form dental restorations, such as inlays, onlays, veneers, partial crowns, crowns, laminating materials and laminates. In each case, lithium disilicate was detected as main crystal phase.

TABLE I Example 1 2 3 4 5 6 7 8 9 10 11 12 Composition wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% wt.-% SiO₂ 73.8 75.0 73.8 72.8 73.8 73.8 73.8 73.8 75.3 76.6 62.1 76.4 Li₂O 15.3 15.5 15.3 15.1 15.3 15.3 15.3 15.3 15.7 15.9 12.9 15.9 P₂O₅ 3.4 3.4 3.4 2.9 3.4 3.4 3.4 3.4 3.4 — 7.0 3.5 Al₂O₃ 3.0 3.0 3.0 3.2 — — — — 3.5 3.0 3.0 — Rb₂O 4.5 3.1 — 6.0 4.5 — — 2.0 2.0 4.5 7.5 4.2 Cs₂O — — 4.5 — — 4.5 4.5 — — — 7.5 — Y₂O₃ — — — — 3.0 3.0 — 3.0 — — — — TiO₂ — — — — — — 3.0 — — — — — MgO — — — — — — — 2.5 — — — — Ag₂O 0.1 — — — Optical properties trans- trans- trans- trans- trans- trans- trans- trans- trans- trans- slightly trans- (after pouring) parent parent parent parent parent parent parent parent parent parent opalescent parent T_(g)/° C. 467 475 470 469 479 481 479 471 471 471 488 475 T_(N)/° C. 480 480 480 500 500 500 500 500 490 490 510 500 t_(N)/min. 10 10 10 10 10 10 10 10 10 10 10 10 T_(C)/° C. 700 700 700 700 700 700 700 700 700 750 680 740 t_(C)/min. 20 20 20 20 20 20 20 20 20 30 20 20 Main crystal lithium lithium lithium lithium lithium lithium lithium lithium lithium lithium lithium lithium phase di- di- di- di- di- di- di- di- di- di- meta- di- silicate silicate silicate silicate silicate silicate silicate silicate silicate silicate silicate silicate Other crystal Li₂SiO₃ Li₃PO₄ Li₂SiO₃ quartz, Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₃PO₄ Li₂SiO₃ lithium — Li₃PO₄, phases Li₂SiO₃ meta- cristo- silicate balite K_(IC)/MPa · m^(1/2) 2.29 2.31 2.08 2.67 2.38 2.45 2.56 2.36 — — — — σ_(B)/MPa 610 — — — — 480 — — — — — — Example 13 14 15 16 Composition wt.-% wt.-% wt.-% wt.-% SiO₂ 70.1 72.8 70.1 70.2 Li₂O 14.5 15.1 19.0 14.5 P₂O₅ 3.2 4.0 3.4 3.2 Al₂O₃ 2.9 3.0 3.0 2.8 Rb₂O 4.3 5.1 4.5 — Cs₂O — — — 4.3 Y₂O₃ — — — — TiO₂ — — — — MgO — — — — Ag₂O — — — — ZrO₂ 5.0 — — 5.0 Optical properties trans- trans- trans- trans- (after pouring) parent parent parent parent T_(g)/° C. 484 469 455 493 T_(N)/° C. 500 500 500 500 t_(N)/min. 10 10 10 10 T_(C)/° C. 660 680 660 660 t_(C)/min. 20 20 20 20 Main crystal lithium lithium lithium lithium phase_(RT-XRD) meta- meta- meta- meta- silicate silicate silicate silicate Other crystal — — lithium lithium phases di- di- silicate silicate 

The invention claimed is:
 1. Lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃, less than 1.0 wt.-% K₂O and is free of Ta₂O₅.
 2. Glass ceramic according to claim 1, which comprises less than 3.8 wt.-% BaO.
 3. Glass ceramic according to claim 1, which comprises K₂O, Na₂O and mixtures thereof in an amount of less than 1.0 wt.-%.
 4. Glass ceramic according to claim 1, wherein the molar ratio of the monovalent metal oxide to Al₂O₃ is at least 0.5.
 5. Glass ceramic according to claim 4, wherein the molar ratio of the monovalent metal oxide to Al₂O₃ is 0.5 to 1.5.
 6. Glass ceramic according to claim 1, which comprises the monovalent metal oxide or mixtures thereof in an amount of from 0.1 to 17.0 wt.-%.
 7. Glass ceramic according to claim 6, which comprises the monovalent metal oxide or mixtures thereof in an amount of from 1.0 to 15.0 wt.-%.
 8. Glass ceramic according to claim 7, which comprises the monovalent metal oxide or mixtures thereof in an amount of from 1.5 to 8.0 wt.-%.
 9. Glass ceramic according to claim 1, which comprises 55.0 to 85.0 wt.-% SiO₂.
 10. Glass ceramic according to claim 1, which comprises 9.0 to 20.0 wt.-% Li₂O.
 11. Glass ceramic according to claim 1, wherein the molar ratio between SiO₂ and Li₂O is from 2.2 to 2.6.
 12. Glass ceramic according to claim 1, which comprises 0 to 12.0 wt.-% P₂O₅.
 13. Glass ceramic according to claim 1, which comprises at least one and preferably all of the following components: Component wt.-% SiO₂ 55.0 to 85.0  Li₂O 9.0 to 17.0 Rb₂O and/or Cs₂O 0.1 to 15.0 P₂O₅  0 to 12.0.


14. Glass ceramic according to claim 1, wherein lithium silicate glass ceramic is excluded which comprises at least 6.1 wt.-% ZrO₂.
 15. Glass ceramic according to claim 1, wherein lithium silicate glass ceramic is excluded which comprises at least 8.5 wt.-% transition metal oxide selected from the group consisting of oxides of yttrium, oxides of transition metals with an atomic number from 41 to 79 and mixtures of these oxides.
 16. Lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃ and less than 1.0 wt.-% K₂O, and which comprises lithium metasilicate as a main crystal phase.
 17. Lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃ and less than 1.0 wt.-% K₂O, and which comprises more than 5 vol.-% lithium metasilicate crystals.
 18. Lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃ and less than 1.0 wt.-% K₂O, and which comprises more than 10 vol.-% lithium metasilicate crystals.
 19. Lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃ and less than 1.0 wt.-% K₂O, and which comprises more than 20 vol.-% lithium metasilicate crystals.
 20. Glass ceramic according to claim 1, which comprises lithium disilicate as a main crystal phase.
 21. Glass ceramic according to claim 1, which comprises more than 10 vol.-% lithium disilicate crystals.
 22. Glass ceramic according to claim 1, which comprises more than 20 vol.-% lithium disilicate crystals.
 23. Glass ceramic according to claim 1, which comprises more than 30 vol.-% lithium disilicate crystals.
 24. Glass ceramic according to claim 1, which has lithium disilicate as a main crystal phase and a fracture toughness, measured as KIC value, of at least about 2.0 MPa·m0.5.
 25. Starting glass, which comprises the components of the glass ceramic according to claim
 1. 26. Lithium silicate glass with nuclei which are suitable for forming lithium metasilicate and/or lithium disilicate crystals, wherein the glass comprises the components of the glass ceramic according to claim
 1. 27. Glass ceramic according to claim 1 wherein the glass ceramic is present in the form of a powder, a granular material, a blank or a dental restoration.
 28. Process for the preparation of the glass ceramic according to claim 1, wherein a starting glass, a glass with nuclei or a glass ceramic with lithium metasilicate as a main crystal phase is subjected to at least one heat treatment in the range of from 450 to 950° C.
 29. Process for the preparation of a lithium silicate glass ceramic which comprises a monovalent metal oxide selected from Rb₂O, Cs₂O and mixtures thereof and comprises less than 5.1 wt.-% Al₂O₃ and less than 1.0 wt.-% K₂O, wherein (a) a starting glass is subjected to a heat treatment at a temperature of from 480 to 560° C. in order to form a glass with nuclei, and (b) the glass with nuclei is subjected to a heat treatment at a temperature of from 700 to 750° C. in order to form the glass ceramic with lithium disilicate as main crystal phase.
 30. Process of using the glass ceramic according to claim 1 as a dental material.
 31. Process according to claim 30, wherein the glass ceramic is shaped by pressing or machining to a dental restoration. 